The present invention relates to an apparatus for printing on opposite surfaces of a strip of printing paper using two electrophotographic printing units each adapted to operate using a light beam, e.g., a laser light beam, wherein a high quality printing operation is performed on each of the opposite surfaces of the printing paper.
To facilitate understanding of the present invention, a conventional electrophotographic printing unit adapted to operate using a laser light beam will be described briefly below with reference to FIG. 4. The conventional electrophotographic printing unit includes a charger 1, a photosensitive drum 2, a light beam 3 (e.g., a laser light beam), a developing portion 4, a first tractor 5 for continuously conveying a strip of printing paper, a second tractor 6 for continuously conveying the printing paper, a strip of printing paper 7 having a series of perforations formed along opposite sides thereof, a preheating plate 11 (heated to a temperature of about 100.degree. C.) for fixing developed toner images, a heat roll 12 (heated to a temperature of about 100.degree. C.) for fixing the toner images, a back-up roll 13 for squeezing the printing paper 7 against the heat roll 12, a pair of puller rollers 14 for pulling the printing paper 7, a stacker 15 for receiving the printing paper 7 discharged from the puller rollers 14, an image transferring portion 17 for transferring the developed toner images onto the printing paper 7, a lower retractor 19, an upper retractor 20, and a retractor spring 21.
With the conventional electrophotographic printing unit constructed as described above, while the photosensitive drum 2 is electrically charged by the charger 1 while receiving a laser light beam 3 emitted in the arrow-marked direction as shown in FIG. 4, each character written on the photosensitive drum 2 in the form of a latent image by the laser light beam 3 is developed by the developing portion 4. Subsequently, as a strip of printing paper 7 is delivered from the first tractor 5 and conveyed to the second tractor 6 via the retractor spring 21, the lower retractor 19, and the upper retractor 20, the developed toner images (i.e., the developed toner characters) on the photosensitive drum 2 are transferred onto the printing paper 7 with the aid of the transferring portion 17.
To assure that the developed toner images are uniformly transferred onto the whole surface of the printing paper 7, the latter is spaced away from the photosensitive drum 2 a predetermined distance (0.5 mm or less) with the aid of the lower and the upper retractor 19, 20. After the printing paper 7 is conveyed further by the second tractor 6, it is sufficiently heated by the preheated plate 11 and then passed between the heat roll 12 and the back-up roll 13, causing the developed toner images to be fixed onto the printing paper 7. Once the fixing operation is completed, the printing paper 7 is received in the stacker 15 with the aid of the puller rollers 14.
However, it is anticipated that, on the assumption that two substantially similar electrophotographic printing units constructed in the above-described manner are used for printing operations, malfunctions will arise which will undesirably degrade the quality of the printing operation. This occurs especially when printing on the reverse surface of the printing paper by the second electrophotographic printing unit after the paper is turned over from the front surface upon completing of the printing operation in the first electrophotographic printing unit.
More specifically, when the developed toner images are fixed onto the printing paper with the aid of a preheating plate and a heat roll in the first electrophotographic printing unit, the printing paper is thermally damaged to a certain extent, causing it to be warped due to local expansion and contraction of the printed paper. This results in the flatness of the printing paper being degraded by a quantity ranging from 0.5 to 1.0 mm. In addition, when the developed toner images are transferred onto the printing paper from a photosensitive drum of the second electrophotographic printing unit, a portion of the developed toner images will probably be incorrectly transferred onto the printing paper due to the fact that because of the aforementioned warpage of the printing paper, it is practically difficult to properly hold the printing paper away from the photosensitive drum in the spaced relationship with a predetermined distance kept therebetween. In other words, a portion of the developed toner images representing certain characters is not printed on the printing paper.
Additionally, since the toner images fixed in the first electrophotographic printing unit come in contact with a preheating plate and a back-up roll in the second electrophotographic printing unit, the fixed toner will probably become molten by the preheating plate with the result that the molten toner is parted away from one surface of the printed paper or it oozes onto the opposite surface of the paper. Thus, a portion of the printed images is not visually recognized. Further, there may arise another problem wherein the surfaces of the preheating plate and the back-up roll are undesirably contaminated with the molten toner.